Field of Invention
The present invention relates generally to RF circuits and more particularly to a circuit that transforms a differential output signal received from a differential power amplifier into a single-ended output signal, where the impedance applied to the differential power amplifier by the circuit can be tuned for multiple operating modes so that a single differential power amplifier can be used for several operating modes operating within a given frequency band.
Related Art
FIG. 1 illustrates a prior art RF circuit 100 coupled to an antenna 110. The circuit 100 comprises a number of transit paths 120 and an antenna switch 130 that couples the antenna 110 to the several transit paths 120. Each transit path 120 includes a power amplifier 140 and a receiver amplifier 150 alternately joined to the antenna switch 130 by a duplexer 160. Within the antenna switch 130, each transit path 120 is switchably coupled to the antenna 110 by a series switch 170 and optionally also switchably coupled to ground by a parallel switch 180.
As shown in FIG. 1, when one transit path 120 is coupled to the antenna 110 (said to be in the ON mode), the series switch 170 for that transit path 120 is closed while the parallel switch 180 is open, whereas the switches 170, 180 are reversed for all other transit paths 120 (OFF mode). While the antenna switch 130 will work with only the serial switches 170, the addition of the parallel switches 180 provides greater isolation by grounding the transit paths 120 that are not in use.
In a cellular telephone, the various transit paths 120 are provided to accommodate different operating modes, where each operating mode is a different combination of frequency band and power level, where the telephone can switch between operating modes depending on changes in the country, the operator, the traffic, and so forth. As shown in FIG. 1, having multiple operating modes can be achieved by using as many power amplifiers 140 as the number of operating modes such that every power amplifier 140 is optimized for its specific frequency band and power level.